The present invention relates to sodium percarbonate particles having a single-layer coating comprising a core substantially consisting of sodium percarbonate and a coating layer enclosing the core and firmly adhering thereto. Despite the use of a single coating material, the coated sodium percarbonate particles are distinguished by very good storage and ensiling properties. The invention moreover relates to the use of the coated sodium percarbonate particles in detergents, cleaning agents and bleaches.
Sodium percarbonate (2 Na.sub.2 CO.sub.3 .multidot.3 H.sub.2 O.sub.2) is used as an active oxygen component in detergents, bleaches and cleaning agents. Due to the unsatisfactory storage stability of the sodium percarbonate in warm/moist surroundings and in the presence of certain detergent and cleaning agent components, sodium percarbonate must be stabilized against the loss of active oxygen (Oa). An essential principle of stabilization involves encasing the sodium percarbonate particles in a coating of components having a stabilizing action.
GB patent 174 891 teaches the stabilization of compounds decomposable by the action of weather or catalysts by coating the compounds. Coating of the compounds is achieved by spraying a coating component in liquid form onto the pulverulent material to be stabilized, which is maintained in motion; in order to avoid liquefaction or caking, the material is cooled or dried by means of a stream of air. Per compounds, such as sodium percarbonate, may be coated with water glass using this process. In the case of sodium percarbonate, it is not possible to achieve adequate stabilization with a layer of water glass alone. In connection with coating soda containing water of crystallization, this document also mentions Glauber's salt, namely sodium sulphate decahydrate, as a possible coating component. This document does not make it obvious to coat sodium percarbonate with a layer substantially consisting of sodium sulphate, which is not present in the form of the hydrates thereof or only partially so.
DE-OS 24 17 572 (corresponding to U.S. Pat. No. 4,105,827) discloses a coated sodium percarbonate, wherein the coating substance is a mixed compound formed by crystallization of sodium carbonate or bicarbonate with sodium sulphate. According to DE patent 26 22 610, also corresponding to U.S. Pat. No. 4,105,827, apart from sodium sulphate and sodium carbonate, the single-layer coating additionally contains a sodium silicate. In both processes described in the above-stated documents, an aqueous solution of the constituents of the coating material is sprayed onto sodium percarbonate particles in a fluidized bed, while maintaining a fluidized bed temperature of between 30 and 80.degree. C., wherein a solid coating is formed by vaporization of the introduced water. Despite distinctly improved stability of the sodium percarbonate particles coated in this manner, the active oxygen content still decreases excessively in the event of extended storage in the presence of a detergent powder. It has additionally been found that coated sodium percarbonate particles having a coating, the outermost layer of which consists of soda alone or as the main component of a mixture, exhibit unsatisfactory ensilability in that beds of such products consolidate over extended periods of storage and thus exhibit poor flow behavior and make handling more difficult.
By way of comparison, sodium percarbonate was also coated with soda alone and with sodium sulphate alone using the process of DE-OS 24 17 572. According to Table IV of this document, the resultant coated products exhibit substantially lower stability in moist/warm storage than products which contain in the coating both sodium sulphate and sodium carbonate in the form of a mixed compound. Even though DE-OS 24 17 572 does not state how the sodium percarbonate to be coated was produced, it may be assumed that the sodium percarbonate was obtained by a crystallization process known as of the filing date because production by the technique of fluidized bed spray granulation only subsequently became known.
According to DE-OS 43 15 380, corresponding to U.S. Pat. No. 5,478,488 the coating material of a coated sodium percarbonate consists of a mineral salt mixture substantially consisting of alkali metal sulphate and alkali metal chloride. Although such coated products exhibit satisfactory active oxygen stability and, provided that no alkali metal silicate is additionally present, also an elevated dissolution rate, they are considered disadvantageous due to their content of alkali metal chloride and the associated corrosion hazard. There is no indication in this document of using sodium sulphate as the sole coating material.
According to U.S. Pat. No. 4,325,933, the hygroscopic nature of sodium percarbonate may be reduced and stability increased with a coating of magnesium salts, in particular magnesium sulphate. However, a coating containing solely magnesium salts no longer fulfills current requirements for active oxygen stability. Thus WO 95/02555 (Australian 94 66479) and EP-A 0 623 553 (U.S. Pat. No. 5,462,804) relate to coated sodium percarbonate particles, the coating of which additionally contains, apart from magnesium sulphate and a silicate, soda or an alkali metal salt which can be in the form of carbonates, bicarbonates and sulphates. While a single- or multi-layer coating does indeed give rise to very good active oxygen stability, the necessity of using three different coating components is disadvantageous. Moreover, products having a multi-layer structure and soda in the outermost coating layer have a tendency to cake.
WO 95/15291 (Australian 95 10689) discloses a process for the production of sodium percarbonate particles having improved stability. In this process, sodium percarbonate is brought into contact with a gas having an elevated CO.sub.2 content and moisture, in order to form a continuous sodium bicarbonate layer on the sodium percarbonate particles. A layer of sodium sulphate may additionally be applied onto the sodium bicarbonate layer. The process is performed in a fluidized bed coating apparatus.